Flat-top tunable filter

ABSTRACT

A tunable PLC optical filter having sequentially connected thermally tunable Mach-Zehnder (MZ) interferometers is described. The cascade of MZ interferometers, each having a free spectral ranges matching ITU frequency grid spacing, are tuned so as to have a common passband centered on the frequency of the signal being selected, while having at least one of the stopbands centered on any other ITU frequency. Any other optical channel that may be present at any other ITU frequency is suppressed as a result. Another MZ interferometer in series with the cascade of interferometers including an asymmetric or variable coupler, is tuned to have low transmission at the center frequency of the selected optical channel.

TECHNICAL FIELD

The present invention relates to a tunable optical filter, and inparticular to a flat-top tunable optical filter having cascadedMach-Zehnder interferometers.

BACKGROUND OF THE INVENTION

Optical filters are often used to select at least one optical frequencyband, called a passband, out of an optical frequency spectrum of anoptical signal. A central frequency of the passband of a tunable opticalfilter is adjustable, depending upon a control parameter common to aparticular filter type. For example, for a bulk optic tunable filter,the control parameter can be a filter tilt or a clocking (rotation)angle with respect to an incoming optical beam. For an optical waveguidebased tunable filter such as tunable Mach-Zehnder (MZ) interferometer,the control parameter can be an electrical signal applied to a localizedheater that changes the optical path length of one of its arms, whicheffectively tunes the MZ interferometer.

Tuning range, spectral selectivity, and a level of cross-talksuppression are very important parameters of tunable optical filters. Awide tuning range allows a wide range of optical frequencies to beaccessed and selected by a tunable filter. The spectral selectivityrelates to an ability of the filter to select a narrow frequency band ofa broadband optical signal. Herein, the term “narrow” means small ascompared to a value of the central frequency of the optical signal beingfiltered, for example 1% of the central frequency or less. Finally, thecrosstalk suppression is an ability of the filter to suppress opticalsignals at any other frequency than the frequency of the signal beingselected.

In an optical communications network, optical signals having a pluralityof optical channels with different optical frequencies or wavelengthscalled optical frequency channels or wavelength channels, aretransmitted from one location to another, typically through a length ofoptical fiber. Optical frequency channels can be combined fortransmission through a single optical fiber, whereby the transmissioncapacity of the optical fiber increases many times. Since the opticalfrequency channels can be amplified simultaneously in a single opticalamplifier, the transmission distances are increased, while theassociated transmission costs are considerably reduced.

Tunable optical filters are used in optical communications networks forselecting one or more optical frequency channel out of a plurality ofchannels comprising the optical communications signal. Tunable opticalfilters are also used for system performance monitoring purposes, e.g.for performing a spectral measurement of the entire opticalcommunications signal, including measuring optical noise levels betweenthe neighboring frequency channels. The tunability of the filter allowsany optical frequency component within the tuning range of the filter tobe selected for subsequent detection and/or signal level measurement.Ideally, a tunable filter has excellent crosstalk suppression, sincepoor crosstalk suppression leads to undesired “leaking” of the opticalchannels being suppressed, thus impairing the signal level measurementsand/or detection and decoding of the selected signal.

U.S. Pat. No. 5,596,661 entitled “Monolithic Optical Waveguide Filtersbased on Fourier Expansion”, issued to Henry et al., and incorporatedherein by reference, teaches a planar lightwave circuit (PLC) opticalfilter having a chain of optical couplers linked by different delayswith a transfer function equal to the sum of the contribution from eachoptical path, with each contribution forming a term in a Fourier serieswhose sum forms the optical output. Detrimentally, the optical filter ofHenry et al. is not tunable.

U.S. Pat. No. 6,208,780 entitled “System and Method for OpticalMonitoring”, issued to Li et al., and incorporated herein by reference,teaches a tunable optical filter on a PLC chip using cascaded unbalancedMach-Zehnder (MZ) interferometers. In the tunable filter of Li et al.,successive MZ stages have twice the free spectral range (FSR) as theprevious MZ stages, thereby providing a narrowband optical filter havinga wide tuning range. Unfortunately, the tunable optical filter requiresmany MZ stages, including stages that have to be repeated, to achieve asatisfactory crosstalk suppression.

U.S. Pat. No. 8,340,523 entitled “Tunable optical filter”, issued toShen et al., hereby incorporated by reference herein, teaches a tunableoptical filter on a PLC chip having sequentially connected thermallytunable MZ interferometers having different FSRs. To achieve a highlevel of crosstalk suppression, each of the MZ interferometers is tunedso as to have one passband of each MZ interferometer centered on thecentral frequency of the single frequency channel being selected, and atleast one of the stopbands of the MZ interferometers centered on acentral frequency of each remaining optical frequency channel of theoptical signal. In contrast to the tunable filter taught by Li et al.,the tunable optical filter taught by Shen et al. includes MZinterferometers having FSRs that are an integral number the frequencygrid. The resulting optical filter has a crosstalk that is improved byat least two orders of magnitude relative to the crosstalk performanceof the filter disclosed in U.S. Pat. No. 6,208,780.

Notably, the tunable optical filter taught in U.S. Pat. No. 8,340,523has a low insertion loss and Gaussian passband shape. In general, aflat-top passband is preferred to a Gaussian passband, since it providesa wider passband and is less likely to alter the optical signal. Inorder to improve the spectral shape of the passpand, Shen et al.disclose an embodiment having a interleaver stage including first andsecond MZ interferometers. These MZ interferometers are tuned to maximumtransmission at the filter wavelength, and have a FSR that is anintegral number of the frequency grid spacing. While this interleaverstage has been shown to provide a wider passband and a steeperroll-over, the bandpass is still substantially Gaussian-like in shape.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the instant invention, a tunableoptical filter having sequentially connected thermally tunable MZinterferometers with different FSRs includes an additional MZ stage forproviding a substantially flat-top passband shape (i.e., relative to theGaussian-like passband shape associated with U.S. Pat. No. 8,340,523).Advantageously, the MZ in this stage uses optical couplers having acoupling ratio that differs from the conventional 50%/50% ratio (e.g.,used in U.S. Pat. No. 6,340,523), and thus has a substantiallysinusoidal response with relatively low modulation. As a result, thetotal transmission of the filter, which is the sum of the Gaussianresponse of the cascade of MZ interferometers and the sinusoidalresponse of the additional MZ interferometer, will be a flat-topspectrum when the additional stage MZ is tuned to have low and/or orminimum transmission at the filter wavelength.

According to one aspect of the present invention there is provided atunable optical filter comprising: an input port for receiving anoptical signal, the optical signal including a plurality of opticalfrequency channels, each optical frequency channel having a centralfrequency substantially centered at a different frequency ofpredetermined frequency grid having a predetermined grid spacing; anoutput port for transmitting an optical frequency channel selected fromthe plurality of optical frequency channels; a plurality of sequentiallycoupled tunable Mach-Zehnder (MZ) interferometers optically disposedbetween the input port and the output port for isolating the selectedoptical frequency channel from the plurality of optical frequencychannels, each tunable MZ interferometer having a plurality ofequidistantly spaced conterminous frequency passbands and frequencystopbands and having a free spectral range substantially equal to aninteger multiple of the predetermined grid spacing; a first MZinterferometer optically disposed between the input port and the outputport, the first MZ interferometer including first and secondinterferometer arms optically disposed between first and second opticalcouplers, the first optical coupler for directing more than 75% of thelight received at an input of the first MZ interferometer into the firstinterferometer arm, the first and second interferometer arms havingdifferent lengths, and a controller for tuning the plurality ofsequentially coupled MZ interferometers to have one passband of each MZinterferometer centered on the central frequency of the selected opticalfrequency channel, and to have at least one of the stopbands of the MZinterferometers centered on the central frequency of each remainingoptical frequency channel of the optical signal, so as to suppress eachsaid remaining optical frequency channel of the optical signal, and fortuning the first MZ interferometer to have low transmission at thecenter frequency of the selected optical frequency channel.

According to another aspect of the present invention there is provided amethod of filtering an optical signal comprising: passing an opticalsignal through a tunable optical filter, the optical signal including aplurality of optical frequency channels, each optical frequency channelhaving a central frequency substantially centered at a differentfrequency of predetermined frequency grid having a predetermined gridspacing, the tunable optical filter including: a plurality ofsequentially coupled tunable Mach-Zehnder (MZ) interferometers forselecting an optical frequency channel from the plurality of opticalfrequency channels, each tunable MZ interferometer having a plurality ofequidistantly spaced conterminous frequency passbands and frequencystopbands and having a free spectral range substantially equal to aninteger multiple of the predetermined grid spacing; a first MZinterferometer optically coupled to the plurality of sequentiallycoupled tunable MZ interferometers, the first MZ interferometerincluding first and second interferometer arms optically disposedbetween first and second optical couplers, the first optical coupler fordirecting more than 75% of the light received at an input of the firstMZ interferometer into the first interferometer arm, the first andsecond interferometer arms having different lengths; and a controller;and tuning the plurality of sequentially coupled MZ interferometers tohave one passband of each MZ interferometer centered on the centralfrequency of the selected optical frequency channel, and to have atleast one of the stopbands of the MZ interferometers centered on thecentral frequency of each remaining optical frequency channel of theoptical signal, so as to suppress each said remaining optical frequencychannel of the optical signal; and tuning the first MZ interferometer tohave low transmission at the center frequency of the selected opticalfrequency channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a tunable flat-top optical filter inaccordance with one embodiment of the instant invention;

FIG. 2 is an optical diagram of an unbalanced Mach-Zehnder (MZ)interferometer having symmetric couplers;

FIG. 3 is a transmission spectrum of the unbalanced MZ interferometer ofFIG. 2;

FIGS. 4A and 4B are diagrams illustrating the principle of cross-talksuppression using a cascade of unbalanced MZ interferometers;

FIG. 5 is an optical diagram of an unbalanced MZ interferometer havingasymmetric couplers;

FIG. 6 shows transmission spectra simulated for a cascade of fifteenunbalanced MZ interferometers having symmetric couplers;

FIG. 7 shows transmission spectra simulated for a single unbalanced MZinterferometers having asymmetric couplers;

FIG. 8 shows transmission spectra simulated for a tunable optical filterincluding a cascade of fifteen unbalanced MZ interferometers havingsymmetric couplers in optical series with a single unbalanced MZinterferometer having asymmetric couplers;

FIG. 9 shows a plot of the 1 dB passband bandwidth and the penalty lossfor the tunable optical filter discussed with reference to FIG. 8, as afunction of coupling ratio of the single unbalanced MZ interferometershaving asymmetric couplers;

FIG. 10 is a schematic diagram of a tunable flat-top optical filter inaccordance with another embodiment of the instant invention, wherein thecascade of unbalanced MZ interferometers having symmetric couplers isdisposed after the single unbalanced MZ interferometer having asymmetriccouplers;

FIG. 11 is a schematic diagram of a tunable flat-top optical filter inaccordance with another embodiment of the instant invention, including ashutter for hitless operation;

FIG. 12 is a schematic diagram of a tunable flat-top optical filter inaccordance with another embodiment of the instant invention, includingtwo unbalanced MZ interferometers having asymmetric couplers;

FIG. 13 is an optical diagram of the two unbalanced MZ interferometershaving asymmetric couplers;

FIG. 14 is a schematic diagram of a tunable flat-top optical filter inaccordance with another embodiment of the instant invention, including asingle unbalanced MZ interferometer, wherein the couplers are MZvariable couplers (VC);

FIG. 15 is an optical diagram of the single unbalanced MZinterferometer, wherein the couplers are MZ VC;

FIG. 16 is a top view of a flat-top tunable optical filter in accordancewith one embodiment of the instant invention, wherein the filter isintegrated on a single PLC chip in a plurality of parallel rows withoptical fibers optically coupling the different rows; and

FIG. 17 is an electrical circuit block diagrams for thermal control ofthe flat-top tunable filter of FIG. 16.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of a flat-top optical filter according toone embodiment of the instant invention. The flat-top filter 1 includesa cascade of Mach-Zehnder (MZ) interferometers 10 connected in serieswith another MZ interferometer 15. Each MZ interferometer in the cascadeof MZ interferometers 10 is commonly referred to as a stage, a MZ stage,and/or a filter stage.

Referring to FIG. 2, an optical diagram of a MZ interferometer 20, whichrepresents any one of the MZ interferometers in the cascade of MZinterferometers 10, is shown. The MZ interferometer 20 has twowaveguides 21 and 22 brought into close proximity to each other at 50%,or 3-dB, evanescent coupler regions 23 and 24, thereby forming two arms25 and 26. The arms 25 and 26 have a localized heater 27 and 28,respectively, for heating the arms 25 and 26, thereby tuning the MZinterferometer 20 by changing relative optical length of these arms. Ingeneral, the tuning range of the heaters will be at least onewavelength, or 27 c in optical phase units. The MZ interferometer 20 isan unbalanced MZ interferometer, meaning that the optical lengths of thearms 25 and 26 differ from each other by more than a few microns, e.g.more than 10 microns. Ports 29A and 29B at the ends of the waveguides 21and 22 are used as input and output ports of the MZ interferometer 20,respectively. Alternatively, the opposite ends of correspondingwaveguides 21 and 22 are used as input or output ports. Notably, sincethe directional couplers 23, 24 are symmetric couplers (i.e., provide a50%/50% coupling ratio), maximum modulation spectrum for the bar stateis provided (e.g., at the output port 29B).

Turning to FIG. 3, a typical transmission spectrum of the MZinterferometer 20 between the ports 29A and 29B thereof is shown. Solidline 32 denotes the transmission function T (f), wherein f is opticalfrequency. The MZ interferometer 20 has a plurality of equidistantlyspaced conterminous frequency passbands 34 and frequency stopbands 36,wherein in any frequency passband 34, the transmission T is equal to orhigher than a threshold value of transmission T_(TH), and in anyfrequency stopband 36, the transmission T is lower than the thresholdvalue T_(TH). As the local heaters 27 and 28 are activated, the relativetemperature of the arms 25 and 26 changes, which shifts its transmissionfunction T(f) as is shown by a dotted line 38. Shifts in both directionsare possible by properly adjusting the relative temperature of the localheaters 27 and 28. The spacing in optical frequency or wavelengthbetween two successive optical intensity maxima is referred to as thefree spectral range (FSR).

In general, the cascade of MZ interferometers 10 will include aplurality of MZ interferometers having different FSRs. When the FSR ofeach of the MZ interferometers 10 is selected in dependence upon thefrequency grid spacing of the optical signal to be filtered (e.g., theInternational Telecommunications Union (ITU) frequency grid), thecascade of MZ interferometers 10 can function as a filter. For example,the cascade of MZ interferometers 10 functions as a filter when the MZinterferometers are tuned so as to have a common passband centered onthe frequency of the signal being selected, and at least one of thestopbands centered on any other ITU frequency. As a result, any otheroptical channel that is present at any other ITU frequency issuppressed.

For example, referring to FIG. 4A, the spectral shape of a typicaloptical signal 40 in an optical communications network is shown. Theoptical signal 40 includes equidistantly spaced optical frequencychannels 41 to be suppressed, and an optical frequency channel 42 to beselected. The optical signal is input into the input port of the cascadeof MZ interferometers 10, which for exemplary purposes is illustrated asa plurality of MZ interferometers 43, including sequentially coupledtunable MZ interferometers 45A to 45E. Each MZ interferometer 45A to 45Ehas a plurality of equidistantly spaced conterminous frequency passbands44A to 44E and frequency stopbands 46A to 46E, corresponding to thepassbands 34 and stopbands 36 of FIG. 3. To achieve a high level ofcrosstalk suppression, the MZ interferometers 45A to 45E are tuned so asto have one passband 44A to 44E of each MZ interferometer 45A to 45Ecentered on the central frequency of the single frequency channel 42being selected, while having at least one of the stopbands 46A to 46E ofthe MZ interferometers 45A to 45E centered on a central frequency ofeach remaining optical frequency channel 41 of the optical signal, so asto suppress each said remaining optical frequency channel 41 as shownwith dashed arrows 49, while selecting the optical frequency channel 42as shown with a dashed arrow 48. Accordingly, the plurality of MZinterferometers 43 functions as an optical filter.

As discussed above, the optical frequency channels 41 and 42 arecentered at on a frequency grid (e.g. a 100 GHz ITU frequency grid or a50 GHz ITU frequency grid). Notably, the phrase “centered on a frequencygrid” refers to being substantially centered in that the channels maydeviate somewhat from the exact grid frequencies, according to typicaltolerances of corresponding transmitters, as is appreciated by thoseskilled in the art. The optical filter 43, tuned as shown in FIG. 4A,can be used for selecting the optical frequency channel 42 for adding ordropping at a network node, or it can be used simply to measure opticalpower of the channel 42.

Turning now to FIG. 4B, a diagram illustrating the operation of theoptical filter 43 for measuring optical signal-to-noise ratio (SNR) ofthe single optical frequency channel 42 of the plurality of the opticalfrequency channels 41 of the optical signal 40 is illustrated. At afirst step, the optical filter 43 is tuned as shown in FIG. 4A, and theoptical power of the channel 42 is measured. Then, the optical filter 43is tuned so as to center one passband 44A to 44E of each tunable MZinterferometer 45A to 45E on a frequency disposed substantially in themiddle between the central frequency of the optical frequency channel 42and the central frequency of one of the neighboring optical frequencychannels 41. The optical filter 43 is tuned so as to have at least oneof the stopbands 46A to 46E of the MZ interferometers 45A to 45Ecentered on a central frequency of each remaining optical channel 41, tosuppress each said remaining optical frequency channel of the opticalsignal 40. Then, the optical power of a noise signal at an output end ofthe filter 43 is measured, and a ratio is taken of the measured value ofthe optical power of the single optical frequency channel 42 to themeasured value of the optical power of the noise signal. This ratio isthe SNR of the optical frequency channel 42.

To achieve the functionality described in FIGS. 4A and 4B, the tunableMZ interferometers 45A to 45E have their respective free spectral rangessatisfying the following condition:

FSR_(m)=(2^(m-1))*Δf _(ITU),  (1)

wherein m=1 . . . 5 for the interferometers 45A to 45E, and Δf_(ITU) isan ITU grid spacing, for example, a 50 GHz or a 100 GHz grid spacing. Inother words, each MZ has an FSR that is an integer multiple of the ITUgrid spacing.

In addition to designing the FSR of each MZ interferometer in thecascade of MZ interferometers 10 to be an integer multiple of the gridspacing, the FSRs will typically increase or decrease along the chain.For example, in one embodiment, the cascade of MZ interferometers 10 isa nine stage filter for an optical signal on the 50 GHz ITU grid,wherein the FSR of the MZ interferometers in the first and second stagesis 50 GHz, in the third and fourth stages is 100 GHz, in the fifth andsixth stages is 200 GHz, in the seventh stage is 400 Hz, in the eighthstage is 800 GHz, and in the ninth stage is 1600 GHz. In anotherembodiment, the cascade of MZ interferometers 10 is an eight stagefilter for an optical signal on the 100 GHz ITU grid, wherein the FSR ofthe MZ interferometers in the first stage is 6400 GHz, in the secondstage is 3200 GHz, in the third stage is 1600 GHz, in the fourth stageis 800 GHz, in the fifth and sixth stages is 400 GHz, and in the seventhand eighth stages is 200 Hz. In general, the number of stages in thecascade of MZ interferometers 10 will vary between 1 and 20, and moretypically between 3 and 18 depending of the filter requirements. Ingeneral, each MZ interferometer in the cascade of MZs interferometers 10will be tuned to high transmission at the filter wavelength.

Referring to FIG. 5, an optical diagram of the MZ interferometer 15 isshown. The MZ interferometer 15 has two waveguides 51 and 52 broughtinto close to each other to form evanescent coupler regions 53 and 54,thereby forming two interferometer arms 55 and 56. However, rather thanthe 50%/50% coupling ratio provided by couplers 23, 24, these couplingregions are designed to form asymmetric directional couplers 53 and 54.The interferometer arms 55 and 56 have a localized heater 57 and 58,respectively, for heating the arms 55 and 56 thereby tuning the MZinterferometer 15 by changing relative optical length of these arms. TheMZ interferometer 15 is an unbalanced MZ interferometer, meaning thatthe optical lengths of the arms 55 and 56 differ from each other by morethan a few microns, e.g. more than 10 microns. Ports 59A and 59B at theends of the waveguides 51 and 52 are used as input and output ports ofthe MZ interferometer 15. In another embodiment, opposite ends ofcorresponding waveguides 51 and 52 are used as input or output ports.

In contrast to the MZ interferometers in the cascade 10, which are tunedto maximum transmission at the filter wavelength, the MZ interferometer15 typically is tuned to low transmission at the filter wavelength, andmore commonly is tuned to minimum transmission at the filter wavelength.In addition, in contrast to the FSR of the MZ interferometers in thecascade 10, which are typically an integer multiple of the grid spacing,the FSR of the MZ interferometer 15 does not have to be matched to thegrid spacing (e.g., the ITU frequency grid). For example, in someembodiments, the FSR will be smaller or greater than the grid spacing.In general, the FSR of the MZ interferometer will vary typically betweenabout 50% and 150% of the grid spacing, and more typically will bebetween 75% and 125% of the grid spacing. Notably, exceptional resultshave been calculated when the FSR of the interferometer 15 is aboutequal to the grid spacing.

As discussed above, the MZ interferometer 15 includes asymmetricdirectional couplers 53 and 54. In general, the coupling ratio of theasymmetric couplers 53, 54 will be between 75%/25% and 100%/0%. Forexample, in one embodiment, the coupling ratio of each of the couplers53, 54 is 85%/15% so that 85% of the signal goes into the upper arm 55,while 15% goes into the lower arm 56 of the interferometer. Notably,exceptional results have been calculated for coupling ratios close to80%/20%. Since the directional couplers 53, 54 are asymmetric couplers(i.e., with coupling ratios other than the conventional 50%/50%), lowmodulation spectrum in the bar state is provided. More specifically, thetransmission spectrum of the optical signal exiting the MZinterferometer will not correspond to the cosine curve provided by a50%/50% directional coupler, but rather will correspond to a slightlymodified sine curve with a relatively low dynamic range.

The total transmission of the optical filter 1 will be the sum of theGaussian-like response of the cascade of MZ interferometers 10 and thesinusoidal response of the MZ interferometer 15. The response of thecascade of MZ interferometers 10 and MZ interferometer 15 has beenmodeled, wherein the cascade of interferometers 10 includes fifteenstages (i.e., wherein the FSR in the first and second stages is 6400GHz, in the third and fourth stages is 3200 GHz, in the fifth and sixthstages is 1600 GHz, in the seventh and eight stages is 800 GHz, in theninth and ten stages is 400 GHz, in the eleventh and twelfth stages is200 GHz, in the thirteenth and fourteenth stages is 100 GHz, and in thefifteenth stage is 150 GHz), and wherein the MZ interferometer 15 has acoupling ratio of 80%/20% and a FSR of 50 GHz. The optical signal to befiltered is on the 50 GHz ITU grid.

Referring to FIG. 6 there is shown the simulated transmission spectrumof the cascade of interferometers 10, wherein the top half of the figurecorresponds to a zoomed-in view of the bottom half of the figure. Theresulting Gaussian-shaped spectrum has a 1 dB passband bandwidth of 17GHz, a 3 dB passband bandwidth of 29.7 GHz, and a 20 dB passbandbandwidth of 76.2 GHz.

Referring to FIG. 7 there is shown the simulated transmission spectrumof the MZ interferometer 15, wherein the top half of the figurecorresponds to a zoomed-in view of the bottom half of the figure. Theresulting spectrum has a substantially sinusoidal shape, including aplurality of equidistantly spaced optical intensity maxima and minima.As discussed above, MZ interferometer is tuned to minimum transmissionat the filter wavelength (i.e., there is an optical intensity minimumcentered at the filter wavelength). Notably, the spectrum has a lowdynamic range (e.g., about 5 dB) and does not extend to close to zerotransmission.

As discussed above, the total transmission of the optical filter 1 willbe the sum of the Gaussian-like response of the cascade of MZinterferometers 10 and the sinusoidal response of the MZ interferometer15. Referring to FIG. 8, there is shown the simulated transmissionspectrum of the cascade of interferometers 10 in series with the MZinterferometer 15, wherein the top half of the figure corresponds to azoomed-in view of the bottom half of the figure. The resulting spectrumhas a substantially flat-top shape, and has a 1 dB passband bandwidth of34.2 GHz, a 3 dB passband bandwidth of 43.2 GHz, and a 20 dB passbandbandwidth of 77.7 GHz. Accordingly, the 1 dB passband bandwidth hasincreased from 17 GHz to 34.2 GHz, while the 3 dB passband bandwidth hasincreased from 29.7 GHz to 43.2 GHz, thus illustrating that the instantconfiguration provides a wider passband with a steeper roll over,thereby further improving isolation between adjacent optical frequencychannels.

In general, in order to improve the spectral bandshape of a filter it isdesirable to minimize the ratio of the 20 dB bandpass bandwidth to the 1dB bandpass bandwidth. In the above described simulation, using the MZinterferometer 15 changes this ratio from 4.5 (i.e., 76.2 GHz/17 GHz) to2.3 (i.e., 77.7 GHz/34.3 GHz). Accordingly, it is clear that thisconfiguration provides a substantially flat-top transmission spectrum.

Referring again to FIG. 8, there is an approximately 4 dB penalty losscalculated for the center wavelength. In general, the penalty loss willbe dependent, at least in part, on the coupling ratio of the asymmetricdirection couplers. For example, referring to FIG. 9, simulationsindicate that the penalty loss will decrease from about 3 dB at acoupling ratio of 75%/25%, to zero at coupling ratio of 100%/0%. Incontrast, simulations also show that the passband bandwidth increaseswith decreasing coupling ratio (e.g., the bandwidth increases from about17 GHz at a coupling ratio of 100%/0% to about 26 GHz at a couplingratio of 75%/25%, peaking at almost 35 GHz near a coupling ratio of80%/20%). Accordingly, the choice of coupling ratio of the asymmetriccouplers in the MZ interferometer 15 will typically involve a compromisebetween low penalty loss and larger bandwidth. Notably, a coupling ratioof 100%/0% corresponds to the special case wherein all of the lighttravels to the upper arm of the interferometer producing substantiallyzero modulation, and thus a Gaussian output from the filter. In general,the coupling ratio of the asymmetric couplers will be selected such thatthe percent of light going into the upper arm will vary between 75% and100%, and more typically will be between 75% and 90%. As discussedabove, exceptional bandwidth has been predicted when the percentage oflight going into the upper arm of the directional couplers is about 80%.

In the above described embodiment, the optical filter 1 is depicted asincluding a cascade of MZ interferometers 10 followed by a MZinterferometer 15. In other embodiments, additional components areprovided and/or the relative position of the cascade of MZinterferometers 10 and the MZ interferometer 15 is interchanged.

Referring to FIG. 10, there is shown an embodiment of a flat-top tunableoptical filter 1001 in accordance with one embodiment of the instantinvention, wherein the cascade of MZ interferometers 10 follows the MZinterferometer 15.

Referring to FIG. 11, there is shown an embodiment of a flat-top tunableoptical filter 1101 in accordance with one embodiment of the instantinvention, including an optical shutter or variable optical attenuator(VOA) 17. In this embodiment, the optical shutter and/or VOA is providedat the output end of the optical filter 1101 for diverting all or aportion of the output signal away from the output port, and includes abalanced MZ stage having two output ports. The difference in arm lengthbetween the arms of the balanced MZ stage is adjusted with one or moreheaters, to various positions between 0° and 180° out of phase so as toprovide variable attenuation and/or blocking function. Of course,alternative shutter and/or VOA arrangements are possible, as is wellknown in the art. For example, in another embodiment, the opticalshutter and/or VOA 17 is provided closer to the input port of the filter1001. Advantageously, the VOA and/or optical shutter 17 can be used forsuppressing the optical signal carrying all optical frequency channelsby diverting all or a portion of the output signal away from the outputport, for example during tuning of the tunable filter, and thus providesa hitless tunable filter.

In the above described embodiments of the instant invention, a single MZinterferometer 15 is used to provide the substantially flat-top spectralresponse. However, in other embodiments one more additional MZinterferometers having asymmetric couplers are used to further adjustthe spectral bandshape and/or improve flatness.

Referring to FIG. 12, there is shown an embodiment of a flat-top tunableoptical filter 1201 in accordance with one embodiment of the instantinvention, including the cascade of MZ interferometers 10, a first MZinterferometer 13, and a second MZ interferometer 14. Optionally, theoptical shutter or variable optical attenuator (VOA) 17 is provided.

Referring to FIG. 13, an optical diagram illustrating the first 13 andsecond 14 MZ interferometers is shown. The MZ interferometers 13, 14have waveguides 131 and 132 brought close to each other to formevanescent coupler regions 133, 134, 135, and 136. Each directionalcoupler 133, 134, 135, and 136 is an asymmetric directional couplerhaving a coupling ratio between 75%/25% and 100%/0%. Each interferometer13, 14 has localized heaters 138 for heating the interferometer arms,thereby tuning the MZ interferometers 13, 14 by changing relativeoptical length of the interferometer arms. In general, each of the MZinterferometers 13, 14 will be tuned to low transmission at the filterwavelength. The MZ interferometers 13, 14 are unbalanced MZinterferometers, and will typically have a FSR that varies between about50% and 150% of the grid spacing, and more typically will be between 75%and 125% of the grid spacing. For example, in one embodiment each MZstage 13, 14 has an FSR that is about equal to the grid spacing.

Since each MZ stage 13, 14 includes asymmetric directional couplers, theoutput transmission spectrum of each stage will correspond tosubstantially sinusoidal curve. The total transmission of the opticalfilter 1201 will be the sum of the Gaussian-like response of the cascadeof MZ interferometers 10 and the sinusoidal responses of the MZinterferometer 13, 14. However, unlike the single MZ interferometer 15,which is typically tuned to tuned to minimum transmission at the filterwavelength, the filter 1201 is tuned such that each MZ interferometer13, 14 has low transmission at the filter wavelength, and such that anoptical intensity minimum of each MZ is symmetrically offset from thefilter wavelength. For example, in one embodiment the first MZinterferometer 13 is tuned to have a minimum transmission at −10 GHzrelative to the filter frequency, while the second MZ interferometer 14is tuned to have minimum transmission at +10 GHz relative to the filterfrequency. Advantageously, this configuration further improves theflatness of the transmission spectrum and reduces the penalty loss atthe filter wavelength.

In the above described embodiments of the instant invention, the MZinterferometers 15, 13, and 14 have fixed coupling ratios. In otherembodiments, the MZ interferometers 13, 14, are replaced with MZinterferometers having a tunable coupling ratio.

Referring to FIG. 14, there is shown an embodiment of a flat-top tunableoptical filter 1401 in accordance with one embodiment of the instantinvention, including the cascade of MZ interferometers 10 and a MZinterferometer 18 using Mach-Zehnder variable couplers (VC).

Referring to FIG. 15, an optical diagram illustrating the MZinterferometer 18 is shown. The MZ interferometer 18 is an unbalanced MZinterferometer, meaning that the optical lengths of the interferometerarms 1520A, 1520B differ from each other by more than a few microns,e.g. more than 10 microns. Light is directed into the interferometerarms 1520 A,B via a first MZ VC 1530, and transmitted to the output portvia a second MZ VC 1540. Each MZ VC 1530, 1540 is a balanced MZinterferometer having two input/output ports. The FSR of each MZ VC1530, 1540 will typically be much larger than the overall span of theWDM spectrum. In general, the FSR of the MZ interferometer 18 will varybetween about 50% and 150% of the grid spacing, and more typically willbe between 75% and 125% of the grid spacing. For example, in oneembodiment, the FSR of the interferometer 18 is about equal to the gridspacing.

In operation, localized heaters coupled to each of the MZ VC 1530, 1540adjust the relative optical length of the VC interferometer arms toproduce varying degrees of interference, and thus a varying amount oflight into the MZ interferometer arms 1520A, 1520B. In general, thefirst MZ VC 1530 will be adjusted such that the amount of lighttransmitted to the upper arm 1520A varies from 75% up to and including100% of the input light. When 100% of the light is transmitted into theupper arm 1520A, the transmission spectrum of the filter 1401 will havea substantially Gaussian shape. In contrast, when 75% to 90% of thelight is transmitted into the upper arm 1520A, the transmission spectrumof the interferometer 18 will correspond to a modified sine curve with arelatively low dynamic range. As a result, when the localized heaterscoupled to the MZ interferometer arms 1520A, 1520B are used to tune theMZ interferometer 18 to minimum transmission at the filter wavelength,the transmission spectrum of the filter 1401 will have substantiallyflat-top shape.

Advantageously, this configuration provides a MZ interferometer 18,wherein the coupling ratio of the couplers is variable. Accordingly, thelocal heaters coupled to the MZ VC 1530, 1540 are adjusted to provide acoupling ratio within the predetermined range, while the local heaterscoupled to interferometer arms 1520A, 1520B are adjusted to tune theinterferometer, thus providing exceptional flexibility in adjustingand/or optimizing the passband shape. For example, as discussed above,adjusting the local heaters coupled to the MZ VCs 1530, 1540 to providea coupling ratio of 100%/0% provides a Gaussian passband, whereasadjusting the local heaters coupled to the MZ VCs 1530, 1540 to providea coupling ratio of 80%/20% provides a flat-top passband.

Advantageously, the tunable optical filter 1401 has high potential forapplications, such as colorless flex-grid applications, wherein it isdesirable to select the passband shape in dependence on the channelsignal modulation and/or other parameters. In addition, since the MZvariable couplers can provide a coupling ratio of 100%/0%, the need forextra components to provide an optical signal bypass is obviated.

Further advantageously, the tunable optical filter 1401 has no movingparts and is small enough to be placed within a single standardhot-pluggable XFP package. In fact, in each of the above describedembodiments, the tunable optical filters are readily fabricated on asingle planar light waveguide circuit (PLC) chip using methods wellknown in the art. For example, in one embodiment the cascade ofinterferometers 10 and the bandpass flattening MZ interferometers (i.e.,13, 14, 15, and/or 18) are arranged in different sections on a PLC thatare coupled to each other via loop-back sections, sections of opticalfibers, and/or mirrors, as for example discussed in U.S. Pat. No.8,340,523. In each case, the plurality of sequentially connectedthermally tunable MZ interferometers are connected in series such thatthe output port of one stage corresponds to the input port of asubsequent stage. In one embodiment, the tunable optical filtersutilizing PLC technology will include PLC waveguides formed using anaccepted technique, such as titanium diffusion or proton exchange, in asilicon, polymer, or semiconductor layer deposited on a substrate. Forexample in one embodiment, the PLC waveguides are formed using aphotolithography process, wherein a positive or negative photoresistand/or photomask is used to provide the MZ interferometer patterns.Photolithography processes used to fabricate MZ interferometers are wellknown in the art and are not described further herein.

Referring to FIG. 16, there is shown a flat-top tunable optical filteraccording to one embodiment of the instant invention, integrated on asingle PLC chip 180. The filter includes a plurality of sequentiallycoupled tunable MZ interferometers including a cascade of MZinterferometers 185 _(1-3, 5-12), as described with reference to FIG. 2,and another MZ interferometer 190, as described with reference to FIG.5. The plurality of sequentially coupled tunable MZ interferometers arearranged in three sections. Each section on the chip 180 has an inputport 181, 182, or 183, and an output port 186, 187, and 188.Accordingly, after passing through the first plurality ofinterferometers, e.g. the interferometers 185 ₉ to 185 ₁₂, an opticalsignal injected into the input port 181 is routed out of the chip 160 atthe output port 186 via an optical fiber 184 to the second input port182. The second input port 182 enables the remaining optical signal topass through the next plurality of stages, e.g. the stages 185 ₅ to 185₈, after which the optical signal is again routed out of the chip 180 atthe output port 187 via an optical fiber 189 to a third input port 183.The third input port 183 enables the remaining optical signal to passthrough the next plurality of stages, e.g. the stages 185 ₁ to 185 ₃ and190, after which the optical signal is routed to the output port 188.Advantageously, using the loopback fibers 184 and 189 allows for aconsiderable reduction of size of the PLC chip 180.

Referring to FIG. 17, an electrical block diagram of a control circuit230A for thermal control of the tunable filter PLC chip is shown. Thecircuit 230A, which is part of the controller of the filter, has adigital signal processing (DSP) module 231, a digital-to-analogconverter (DAC) 232, an analog-to-digital converter (ADC) 233, a MZinterferometer heaters driver module 234, a compensation heater driver235, a thermal sensor 236, a transimpedance amplifier (TIA) 237, and aphotodiode 238.

In operation, the DSP module 231 controls the amount of heat applied tothe chip 210 by providing a digital control signal to the DAC 232, whichprovides analog control signals to the MZ driver 234 and to thecompensation heater driver 235. The MZ driver 234 generates electricalcurrents for driving local heaters of the chip (e.g., for the cascade ofinterferometers 10 and the bandpass flattening MZs 13, 14, 15, and 18).The CH driver 235 generates an electrical current for driving thecompensation heater 220, which is disposed beneath the chip. In general,the DSP module 231 controls the amount of heat so that the total amountof heat generated by the local heaters and the compensation heater isconstant, so that the temperature of the encased PLC chip does notchange significantly upon tuning of individual MZ stages, thus providinga more stable alignment. The thermal sensor 236 generates an electricalsignal representative of the temperature of the chip. This signal isdigitized by the ADC 233 and, in digital form, is provided to the DSPmodule 231 for correcting the amount of heat generated by one or moreheaters. According to one control method, the DSP module is operable tocorrect the amount of heat generated by the local heaters, not shown inFIG. 23A, so as to reduce dependence of the optical phases of the MZinterferometers on the overall PLC chip temperature. According toanother control method of the present invention, the DSP module isoperable to control the amount of the heat generated, so as to stabilizethe temperature of the chip. Of course, alternative control circuits arepossible, as for example, disclosed in U.S. Pat. No. 8,340,523.

Of course, the above embodiments and applications have been provided asexamples only. It will be appreciated by those of ordinary skill in theart that various modifications, alternate configurations, and/orequivalents will be employed without departing from the spirit and scopeof the invention. For example, while the above embodiments describelocalized heaters for tuning the MZ interferometers, other optical pathlength adjusters are also possible. For example, in other embodiments,the localized heaters are replaced with acoustic, electric-field, orcurrent-based optical path length adjusters. In addition, while theabove described embodiment show the cascade of interferometers 10 andthe band-shaping interferometer 13, 14, 15, 18 being formed on a samePLC chip, it other embodiments, the cascade of interferometers 10 andthe band-shaping interferometer 13, 14, 15, 18 are formed on differentchips. Accordingly, the scope of the invention is therefore intended tobe limited solely by the scope of the appended claims

What is claimed is:
 1. A tunable optical filter comprising: an inputport for receiving an optical signal, the optical signal including aplurality of optical frequency channels, each optical frequency channelhaving a central frequency substantially centered at a differentfrequency of predetermined frequency grid having a predetermined gridspacing; an output port for transmitting an optical frequency channelselected from the plurality of optical frequency channels; a pluralityof sequentially coupled tunable Mach-Zehnder (MZ) interferometersoptically disposed between the input port and the output port forisolating the selected optical frequency channel from the plurality ofoptical frequency channels, each tunable MZ interferometer having aplurality of equidistantly spaced conterminous frequency passbands andfrequency stopbands and having a free spectral range substantially equalto an integer multiple of the predetermined grid spacing; a first MZinterferometer optically disposed between the input port and the outputport, the first MZ interferometer including first and secondinterferometer arms optically disposed between first and second opticalcouplers, the first optical coupler for directing more than 75% of thelight received at an input of the first MZ interferometer into the firstinterferometer arm, the first and second interferometer arms havingdifferent lengths, and a controller for tuning the plurality ofsequentially coupled MZ interferometers to have one passband of each MZinterferometer centered on the central frequency of the selected opticalfrequency channel, and to have at least one of the stopbands of the MZinterferometers centered on the central frequency of each remainingoptical frequency channel of the optical signal, so as to suppress eachsaid remaining optical frequency channel of the optical signal, and fortuning the first MZ interferometer to have low transmission at thecenter frequency of the selected optical frequency channel.
 2. A tunableoptical filter according to claim 1, wherein the first MZ interferometerhas a free spectral range close to the predetermined grid spacing.
 3. Atunable optical filter according to claim 2, wherein each of the firstand second optical couplers is an asymmetric coupler having a couplingratio between 75%/25% and 99%/1%.
 4. A tunable optical filter accordingto claim 2, wherein each of the first and second optical couplers is aMach-Zehnder variable coupler for providing a coupling ratio between75%/25% and 100%/0%.
 5. A tunable optical filter according to claim 2,wherein a transmission spectrum of the first MZ interferometer includesa plurality of equidistantly spaced optical intensity maxima and minima,and wherein the controller is for tuning the first MZ interferometer tohave an optical intensity minimum at the center frequency of theselected optical frequency channel.
 6. A tunable optical filteraccording to claim 3, wherein a transmission spectrum of the first MZinterferometer includes a plurality of equidistantly spaced opticalintensity maxima and minima, and wherein the controller is for tuningthe first MZ interferometer to have an optical intensity minimum at thecenter frequency of the selected optical frequency channel.
 7. A tunableoptical filter according to claim 4, wherein a transmission spectrum ofthe first MZ interferometer includes a plurality of equidistantly spacedoptical intensity maxima and minima, and wherein the controller is fortuning the first MZ interferometer to have an optical intensity minimumat the center frequency of the selected optical frequency channel.
 8. Atunable optical filter according to claim 2, comprising a second MZinterferometer optically disposed between the input port and the outputport, the second MZ interferometer including third and fourthinterferometer arms optically disposed between third and fourth opticalcouplers, the third optical coupler for directing more than 75% of thelight received at an input of the second MZ interferometer into thethird interferometer arm, the third and fourth interferometer armshaving different lengths, wherein a transmission spectrum of the secondMZ interferometer includes a plurality of equidistantly spaced opticalintensity maxima and minima, and wherein the controller is for tuningthe first MZ interferometer to have an optical intensity minimum at afirst frequency and the second MZ interferometer to have an opticalintensity at a second frequency, the first frequency different from thesecond frequency, each of the first and second frequencies offset fromthe center frequency of the selected optical frequency channel by a samemagnitude.
 9. A tunable optical filter according to claim 1, wherein theplurality of sequentially coupled tunable MZ interferometers produce asubstantially Gaussian shaped transmission spectrum, and wherein atransmission spectrum of the first MZ interferometer includes aplurality of equidistantly spaced optical intensity maxima alternatingwith optical intensity minima, and wherein the controller is for tuningthe first MZ interferometer to have at least one optical intensityminimum centered at the center frequency of the selected opticalfrequency channel.
 10. A tunable optical filter according to claim 1,wherein the frequency grid is the ITU frequency grid, and wherein thepredetermined grid spacing is one of 50 GHz and 100 GHz.
 11. A tunableoptical filter according to claim 10, wherein the plurality of the MZinterferometers includes at least 9 stages of MZ interferometersincluding MZ interferometers having an FSR of 100 GHz, 200 GHz, 400 GHz,800 GHz, and 1600 GHz.
 12. A tunable optical filter according to claim11, wherein the first MZ interferometer has an FSR that is about equalto the predetermined grid spacing.
 13. A tunable optical filteraccording to claim 1, wherein the first MZ interferometer has an FSRthat is between about 50% and 150% of the predetermined grid spacing.14. A tunable optical filter according to claim 8, wherein each of thefirst and second MZ interferometers has an FSR substantially equal tothe predetermined grid spacing.
 15. A tunable optical filter accordingto claim 1, comprising an optical shutter optically disposed between theinput port and the output port for suppressing the optical signal duringtuning of said plurality of sequentially coupled MZ interferometers. 16.A tunable optical filter according to claim 1, wherein the plurality ofsequentially coupled MZ interferometers and the first MZ interferometerare integrated on a same planar lightwave circuit (PLC) chip.
 17. Atunable optical filter according to claim 16, comprising a plurality oflocal heaters disposed on a top surface of the PLC chip and coupled tothe controller, for thermally tuning the plurality of sequentiallycoupled MZ interferometers and the first MZ interferometer.
 18. Atunable optical filter according to claim 3, wherein each of the firstand second optical couplers is an asymmetric coupler having a couplingratio of about 80%/20%.
 19. A method of filtering an optical signalcomprising: passing an optical signal through a tunable optical filter,the optical signal including a plurality of optical frequency channels,each optical frequency channel having a central frequency substantiallycentered at a different frequency of predetermined frequency grid havinga predetermined grid spacing, the tunable optical filter including: aplurality of sequentially coupled tunable Mach-Zehnder (MZ)interferometers for selecting an optical frequency channel from theplurality of optical frequency channels, each tunable MZ interferometerhaving a plurality of equidistantly spaced conterminous frequencypassbands and frequency stopbands and having a free spectral rangesubstantially equal to an integer multiple of the predetermined gridspacing; a first MZ interferometer optically coupled to the plurality ofsequentially coupled tunable MZ interferometers, the first MZinterferometer including first and second interferometer arms opticallydisposed between first and second optical couplers, the first opticalcoupler for directing more than 75% of the light received at an input ofthe first MZ interferometer into the first interferometer arm, the firstand second interferometer arms having different lengths; and acontroller; and tuning the plurality of sequentially coupled MZinterferometers to have one passband of each MZ interferometer centeredon the central frequency of the selected optical frequency channel, andto have at least one of the stopbands of the MZ interferometers centeredon the central frequency of each remaining optical frequency channel ofthe optical signal, so as to suppress each said remaining opticalfrequency channel of the optical signal; and tuning the first MZinterferometer to have low transmission at the center frequency of theselected optical frequency channel.
 20. A method according to claim 19,wherein tuning the first MZ interferometer to have low transmission atthe center frequency of the selected optical frequency channel comprisestuning the first MZ interferometer to have an optical intensity minimumat the center frequency of the selected optical frequency channel.